The housing of the blood treatment unit according to the invention holds both a blood pump and a semi-permeable membrane. The blood pump is an impeller pump. The impeller of the impeller pump is surrounded by a partition which divides the housing into two chambers. The semi-permeable membrane forms at least a part of the partition. It is therefore possible for an exchange of matter to take place between the blood which flows through the first chamber of the housing and the dialysis fluid which flows through the second chamber of the housing. The blood treatment unit is intended for an extra-corporeal blood treatment apparatus and has a particularly compact construction.